This invention relates to a method for epitaxial growth of semiconductor materials, such as a III-V semiconductor epitaxially grown on silicon or germanium. The invention also relates to semiconductor devices formed using the disclosed method.
The ability to grow polar semiconductors, such as GaAs or other III-V materials, on non-polar substrates, such as Si or Ge, has been recognized as highly desirable for some time. The III-V semiconductors are well known to exhibit superior performance characteristics, in many respects, as compared to the more common silicon, but there are a number of difficulties associated with processing III-V semiconductor materials. For example, high quality GaAs substrates of large size are not available at practical cost and are very fragile. In contrast, large silicon wafers are relatively sturdy and inexpensive. If GaAs could be readily grown on Si without undue defects, large GaAs working areas would be available for device production. Among other reasons advanced for growing III-V's on Si are: the possibility of integrated circuits using Si technology but with optical components; and the heat sinking of III-V devices grown on Si due to the higher thermal conductivity of Si. Also, oval defect densities are reduced when starting with a Si substrate.
It has been recognized that there are at least two major problems in epitaxially growing GaAs on Si. First, there is a lattice mismatch between Ga and Si (about 4% for this case). This results in a large number of defects in the interface, which could propagate in the epilayer. Second, antiphase disorder results from both Ga and As atoms being able to bond to a given site on the nonpolar (Si) surface. In some regions the growth begins with the Ga plane, and in other regions growth begins with the As plane. Similar problems are present for growth of other polar semiconductors on non-polar Si or Ge. Some of the background prior art techniques directed to these problems will now be summarized.
In early reports of epitaxial III-V layers on elemental Si or Ge substrates, it was found that antiphase domains occurred in these layers grown by vapor phase epitaxy. (See K. Morizane, Journal of Crystal Growth, 38, 249, 1977.) A drawback of using vapor phase epitaxy for these materials is that it requires a relatively high growth temperature. With the advent of molecular beam epitaxy, which is a much lower temperature process, attempts to grow superlattices of Ge and GaAs resulted in antiphase domain formation. (See P. M. Petroff, et al., Journal of Crystal Growth 46, 172, 1979.)
It was shown that GaAs can be successfully grown by molecular beam epitaxy on (100) Ge by using an As priming layer and an initially large As:Ga flux ratio (see Masselink, et al., Appl. Phys. Lett. 45, 457, 1984 and W. I. Wang, Appl. Phys. Lett. 44, 1149, 1984).
In another approach (see Wright, et al., J. Vac. Sci and Technol. 21, 534, 1982), a silicon (211) surface orientation was found effective in suppressing antiphase disorder. However, the (211) surface would not work well in a monolithically integrated GaAs/Si system. Another drawback is that the orientation must be almost exactly (211), thereby leaving little tolerance for misalignment when cutting wafers.
With regard to dislocations resulting from lattice mismatch, one approach was to use a Ge interlayer between {100} silicon and the GaAs (see, for example, Fletcher, et al., Applied Phys. Lett. 44, 967, 1984 and Sheldon, et al., Appl. Phys. Lett. 45, 274, 1984). A drawback of this technique, however, is Ge contamination of the GaAs.
It has been shown that tilting of substrates can be used advantageously when epitaxially depositing GaAs. Hollan, et al. (J. Crystal Growth, 22, 175, 1974) studied vapor phase epitaxy of GaAs on GaAs, with misorientations reaching 5 degrees in all directions around the (001) plane. Regarding GaAs on Si, applicants noted that using substrates tilted away from the (100) plane, such as by 1/2 degree or 2 degrees, resulted in reduction of antiphase domains and obtainment of uniformly good GaAs across the substrate. (See Masselink, et al., Appl. Phys. Lett. 45, 12, 1984). A tilt toward &lt;011&gt; was used by applicants in this work. Windhorn, et al., of Lincoln Laboratory, (Appl. Phys. Lett., 45, 4, 1984) reported on growing GaAs and AlGaAs layers on a Ge-coated Si wafer oriented 2.degree. off {100} toward the &lt;110&gt; direction. Metze, et al. (Appl. Phys. Lett. 45, 10, 1984), also of Lincoln Laboratory, reported GaAs layers grown by molecular beam epitaxy directly on Si (100) substrates oriented 2 degrees off (100), without specifying the direction off tilt. Sheldon, et al. (Appl. Phys. Lett. 45, 3, 1984) used molecular beam epitaxy to grow GaAs on Si substrates with an intermediate Ge layer. The Si substrates used by Sheldon, et al. were heavily doped p-type oriented 2 degrees off the (100) towards the [111]direction.
It is among the objects of the present invention to provide an improved method of epitaxially depositing semiconductor material on a substrate while accommodating lattice mismatch in a way that results in improved material quality.